The Federal Aviation Administration (FAA) reconsidered aircraft fuel tank safety after a series of fuel tank explosions between 1990 and 2001. The National Transport Safety Board (NTSB) added “Explosive Mixture in Fuel tanks in Transport Category Aircrafts” as the number one item in its 1997 “Most Wanted” Transportation Safety list. Some known fuel tanks have a region containing liquid fuel and an ullage region that often contains evaporated fuel (i.e., fuel vapor). With the presence of air, the mixture may exhibit a fuel-air ratio within the ullage and ignition may occur when fuel-air ratio in the ullage lies within a certain range. The lower flammability limit (LFL) is defined as the fuel temperature below which the fuel-air ratio is too lean to ignite. Similarly, the upper flammability limit (UFL) is defined as the fuel temperature above which the fuel-air ratio is too rich to ignite. The fuel-air ratios between the lower flammability limit and the upper flammability limit are flammable.
In the context of the present document, a fuel tank is flammable when fuel temperature is between the Lower and Upper Flammability Limits (LFL, UFL), and a fuel tank is inert when the tank oxygen is below the Inert Limit, as defined by the 14 Code of Federal Regulations § 25.981(b), Appendix N. LFL and UFL are a function of fuel flash point and altitude, while Inert Limit is a function of altitude. The flammability exposure is defined as the length time, or percentage of evaluation time, during which the tank is flammable. Nonflammable ullage exists when the fuel temperature is either outside the range between the LFL and UFL or tank oxygen is below the inert limit. “Inerting” refers to the process of reducing flammability exposure by introducing noncombustible gas into the ullage of a fuel tank so that the ullage becomes nonflammable. “Noncombustible gas” includes oxygen depleted air (often referred to as nitrogen enriched air (NEA)), nitrogen, or other inert gases. The nitrogen can be obtained from cryogenic storage bottles on board the aircraft or produced from the nitrogen in air.
The ullage fuel-air ratio for Jet A fuel is generally outside of the flammability region. However, known conditions exist that may result in Jet A in a fuel tank being flammable. One example includes a rapid reduction in tank ullage pressure after takeoff, such as when the aircraft climbs, during the time before fuel-tank temperature sufficiently decreases during cruise.
FAA regulations require that new and in-service transport aircraft include systems for enhancing the safety of aircraft fuel tanks. For protection against fire/explosion in the fuel tank ullage, several previous systems have been used. Such systems may be known by a number of designations including, but not limited to, On-Board Inert Gas Generation System (OBIGGS), Flammability Reduction System (FRS), Fuel Tank Inerting System (FTIS), etc. OBIGGS is applied most often to military aircraft that require much more stringent inerting requirements. FRS and FTIS are applied most often to commercial aircraft that use less stringent requirements for flammability reduction. OBIGGS is used in many commercial and cargo airplanes and military aircraft. A commonality among the systems involves reducing the oxygen content of fuel tank ullage by feeding noncombustible gas into the fuel tank. Often, the systems produce nitrogen-enriched air (NEA) for the noncombustible gas, such as with a Nitrogen Generation System (NGS).
Inerting systems used to produce noncombustible gas may rely on pressure swing absorption and desorption from media as a separation mechanism, or diffusion through membranes as another separation mechanism, to remove oxygen. In known inerting systems with hollow fiber membranes, compressed air enters the bore of the hollow fiber and oxygen permeates through the hollow fiber walls, where oxygen permeates more readily than nitrogen does. The oxygen permeate is collected and exhausted overboard. The remaining nitrogen-enriched retentate flows through the bore and is collected at the air separation module product gas outlet for distribution to aircraft fuel tanks. Unfortunately, service life of the air separation module might be limited by the materials used in construction of the module.
Accordingly, known ullage inerting systems can be expensive, complex, and increase the weight of the aircraft. It will be appreciated that ullage inerting systems that decrease system cost, simplify known systems, or decrease weight of the aircraft would be beneficial.